WO2013035159A1 - Exhaust gas purification device for internal combustion engine - Google Patents

Abstract

In order that, when another catalyst is provided on the upstream side from an NOx selective reduction catalyst, both the catalysts are properly recovered from sulfur poisoning, the present invention is provided with: an NH₃ generation catalyst which is provided in an exhaust passage of an internal combustion engine and generates NH₃; an NOx selective reduction catalyst which is provided in the exhaust passage downstream from the NH₃ generation catalyst and selectively reduces NOx; an upstream-side recovery unit which recovers the NH₃ generation catalyst from sulfur poisoning; and a downstream-side recovery unit which recovers the NOx selective reduction catalyst from sulfur poisoning after the NH₃ generation catalyst is recovered from the sulfur poisoning by the upstream-side recovery unit.

Description

Exhaust gas purification device for internal combustion engine

The present invention relates to an exhaust purification device for an internal combustion engine.

2. Description of the Related Art There is known a technique for arranging a NOx storage reduction catalyst (hereinafter also referred to as an NSR catalyst) in an exhaust passage of an internal combustion engine. The NSR catalyst stores NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and reduces the stored NOx when the oxygen concentration of the inflowing exhaust gas decreases and a reducing agent is present.

This NSR catalyst also stores sulfur oxides (SOx) produced by combustion of sulfur components contained in the fuel, similar to NOx. The stored SOx is less likely to be released than NOx and accumulates in the NSR catalyst. This is called sulfur poisoning. This sulfur poisoning reduces the NOx purification rate of the NSR catalyst, so it is necessary to perform a sulfur poisoning recovery process at an appropriate time. This sulfur poisoning recovery process is performed by causing the NSR catalyst to have a high temperature and the exhaust gas with a reduced oxygen concentration to flow through the NSR catalyst.

In this way, when the oxygen concentration is reduced to a rich air-fuel ratio at the time of recovery from sulfur poisoning, H ₂ S may flow out downstream of the NSR catalyst and an odor may be generated.

Here, a technique for suppressing the emission of H ₂ S by performing a lean spike while setting the target air-fuel ratio to a rich air-fuel ratio at the time of sulfur poisoning recovery processing and setting the lean air-fuel ratio for a short period at a predetermined time. Is known (for example, see Patent Document 1). In this technique, the interval at which the lean spike is performed is increased as the amount of the sulfur component deposited decreases.

Further, by supplying secondary air during the sulfur poisoning recovery process, techniques for oxidizing the H 2 _S is known (e.g., see Patent Document 2.).

In addition, a technique is known in which the throttle is opened during deceleration fuel cut to supply air to the catalyst, thereby bringing the catalyst into an oxidized state and suppressing the generation of H ₂ S (see, for example, Patent Document 3).

Further, by increasing the time of the fuel cut operation, thereby suppressing the generation of H 2 _S it is known (e.g., see Patent Document 4.).

Further, there is known a technique for performing control so that the flow rate of air flowing into the exhaust purification catalyst is larger than that during idling when the exhaust purification catalyst is in a state of generating a strange odor when performing fuel cut control. (For example, refer to Patent Document 5).

Incidentally, a selective reduction type NOx catalyst (hereinafter also referred to as an SCR catalyst) can be provided downstream of the NSR catalyst or the three-way catalyst in which sulfur poisoning occurs. This SCR catalyst is a catalyst that selectively reduces NOx with a reducing agent. Then, sulfur poisoning may occur in the SCR catalyst due to H ₂ S flowing out from the NSR catalyst. Here, the prior art does not mention the sulfur poisoning recovery process of both catalysts when the SCR catalyst is provided downstream of the NSR catalyst. For this reason, there exists a possibility that sulfur poisoning recovery may not be performed appropriately. Therefore, there is a possibility that stinky been released H _{2 S} to the atmosphere. In addition, the NOx purification rate may be reduced.

JP 2004-108176 A JP 2005-248869 A JP 2006-348874 A JP 2006-349025 A JP 2007-092609 A

The present invention has been made in view of the above-described problems, and its purpose is to appropriately prevent sulfur poisoning of both catalysts when other catalysts are provided upstream of the selective reduction type NOx catalyst. It is to recover.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention comprises:
And NH ₃ synthesizing catalyst to produce a NH ₃ provided in an exhaust passage of an internal combustion engine,
A selective reduction type NOx catalyst that is provided in an exhaust passage downstream of the NH ₃ production catalyst and selectively reduces NOx;
An upstream recovery portion for recovering sulfur poisoning of the NH ₃ production catalyst;
A downstream recovery unit that recovers sulfur poisoning of the selective reduction type NOx catalyst after the sulfur recovery of the NH ₃ production catalyst is recovered by the upstream recovery unit;
Is provided.

The NH ₃ generation catalyst is, for example, a catalyst that generates NH ₃ by reacting H ₂ or HC with NO. The NH ₃ production catalyst is a catalyst that is poisoned by the sulfur component in the exhaust gas. Here, when the upstream recovery unit recovers the sulfur poisoning of the NH ₃ production catalyst, H ₂ S is generated depending on conditions. This H ₂ S flows into the selective reduction type NOx catalyst provided downstream. This H ₂ S is adsorbed by the selective reduction type NOx. Thereby, sulfur poisoning also occurs in the selective reduction type NOx catalyst. That is, the amount capable of adsorbing NH ₃ decreases according to the amount of adsorption of H ₂ S.

The downstream recovery unit recovers the sulfur poisoning of the selective reduction type NOx catalyst after the upstream recovery unit recovers the sulfur poisoning of the NH ₃ production catalyst. That is, H ₂ S flows out from the NH ₃ production catalyst, and the H ₂ S is adsorbed by the selective reduction NOx catalyst, and then the sulfur poisoning of the selective reduction NOx catalyst is recovered. Here, at the time of sulfur poisoning recovery of the NH ₃ synthesizing catalyst, the temperature of the NH ₃ synthesizing catalyst is increased. Thereby, the temperature of the selective reduction type NOx catalyst also rises. Therefore, after the sulfur poisoning recovery of the NH ₃ production catalyst is performed, the sulfur poisoning recovery of the selective reduction type NOx catalyst is performed, so that the sulfur poisoning recovery is performed in a state where the temperature of the selective reduction type NOx catalyst is high. Can do. Thereby, recovery of sulfur poisoning of the selective reduction type NOx catalyst can be promoted.

In addition, since the selective reduction type NOx catalyst adsorbs H ₂ S generated when the sulfur poisoning of the NH ₃ catalyst is recovered, the generation of a strange odor due to H ₂ S when the sulfur poisoning of the NH ₃ catalyst is recovered is suppressed. it can. After that, when the air-fuel ratio of the exhaust gas passing through the selective reduction type NOx catalyst is a lean air-fuel ratio, H ₂ S is discharged as SOx, so that the generation of a strange odor is suppressed. Further, by recovering the sulfur poisoning of the NH ₃ production catalyst, the NOx occlusion ability is restored and the NH ₃ production ability is restored, so that NH ₃ can be supplied as a reducing agent to the selective reduction type NOx catalyst. It becomes possible. Thereby, the NOx purification rate can be increased.

Note that the timing at which the downstream recovery unit recovers the sulfur poisoning of the selective reduction type NOx catalyst may be immediately after the upstream recovery unit recovers the sulfur poisoning of the NH ₃ production catalyst. Also, this time, due to the effect obtained by recovering the sulfur poisoning of the NH ₃ synthesizing catalyst, when the temperature of the selective reduction type NOx catalyst is higher than before starting the recovery of sulfur poisoning of the NH ₃ synthesizing catalyst It is good. Furthermore, effects of recovering the sulfur poisoning of the NH ₃ synthesizing catalyst may be time remaining in NH ₃ synthesizing catalyst or NOx selective reduction catalyst. Recovery of sulfur poisoning by the upstream recovery unit and recovery of sulfur poisoning by the downstream recovery unit may be performed continuously, or may be performed at a certain interval.

In the present invention, the downstream recovery section, the temperature of the selective reduction type NOx catalyst rises when the upstream recovery unit recovers the sulfur poisoning of the NH ₃ synthesizing catalyst is, the NH ₃ produced You may suppress falling after recovering the sulfur poisoning of a catalyst.

For example, the downstream recovery unit maintains the temperature of the NH ₃ synthesizing catalyst or NOx selective reduction catalyst when the NH ₃ recovery of sulfur poisoning of the generated catalyst is completed. Further, for example, downstream recovery section, the temperature of the NH ₃ synthesizing catalyst or NOx selective reduction catalyst increases when allowed to recover sulfur poisoning of the NH ₃ synthesizing catalyst is, recover sulfur poisoning of the NH ₃ synthesizing catalyst Increase the time until the temperature drops to the temperature before the start. Further, for example, downstream recovery section, NH ₃ than the temperature of the NH ₃ synthesizing catalyst or NOx selective reduction catalyst when sulfur poisoning of generating catalyst were allowed to recover, to increase the subsequent temperature.

Here, the higher the temperature of the selective reduction type NOx catalyst, the more the recovery of sulfur poisoning of the selective reduction type NOx catalyst can be promoted. In addition, the higher the temperature of the NH ₃ production catalyst, the higher the temperature of the selective reduction type NOx catalyst. Therefore, recovery of sulfur poisoning of the selective reduction type NOx catalyst can be promoted. Therefore, after the sulfur poisoning of the NH ₃ production catalyst is recovered, the reduction of the temperature of the NH ₃ production catalyst or the selective reduction type NOx catalyst is suppressed, thereby recovering the sulfur poisoning of the selective reduction type NOx catalyst. Can be promoted. Further, at this time, by setting the stoichiometric air-fuel ratio or the lean air-fuel ratio, H ₂ S changes to SO x, so that the release of H ₂ S into the atmosphere can be suppressed.

Further, in the present invention, the downstream recovery unit is configured to reduce the amount of oxygen flowing into the selective reduction NOx catalyst during a predetermined period after the sulfur recovery of the NH ₃ production catalyst is recovered by the upstream recovery unit. The sulfur poisoning can be recovered by increasing the amount of oxygen flowing into the selective catalytic reduction NOx catalyst after the predetermined period.

This predetermined period is a period during which H ₂ S is adsorbed on the selective reduction type NOx catalyst, or a period required to decrease until the H ₂ S adsorbed by the selective reduction type NOx catalyst falls within an allowable range. Also good. Further, the predetermined period may be a period required for the H ₂ S emission amount to be within the allowable range. Further, the start of the predetermined period may be immediately after the sulfur poisoning of the NH ₃ generation catalyst is recovered by the upstream recovery unit, or may be a time point after the sulfur poisoning of the NH ₃ generation catalyst is recovered. Good. Here, the greater the amount of oxygen flowing into the selective reduction type NOx catalyst, the easier it is for H ₂ S to change to SOx. For this reason, generation | occurrence | production of a strange odor can be suppressed by increasing the amount of oxygen which flows in into a selective reduction type | mold NOx catalyst. Further, the selective reduction type NOx catalyst can adsorb NH ₃ at an early stage.

Note that the total amount of oxygen flowing into the selective reduction NOx catalyst during a predetermined period after the recovery of sulfur poisoning by the upstream recovery unit is completed is the amount of oxygen flowing into the selective reduction NOx catalyst during the same period thereafter. You may make it become more than the sum total. Further, the amount of oxygen per unit time that flows into the selective reduction NOx catalyst during a predetermined period after the recovery of the sulfur poisoning by the upstream side recovery unit is completed, and thereafter the unit time that flows into the selective reduction NOx catalyst You may make it become more than the amount of oxygen per hit.

Further, in the present invention, the downstream recovery unit can make the air-fuel ratio of the exhaust gas higher in the predetermined period than in the predetermined period.

That is, by increasing the air-fuel ratio of the exhaust gas, a large amount of oxygen can flow into the selective reduction type NOx catalyst.

Further, in the present invention, the downstream recovery unit can increase the intake air amount of the internal combustion engine in the predetermined period more than in the predetermined period.

That is, by increasing the amount of intake air, even if the air-fuel ratio is the same, a large amount of oxygen can flow into the selective reduction NOx catalyst.

In the present invention, the downstream recovery section, from the NH ₃ and sulfur poisoning of the generated catalyst is recovered by the upstream recovery unit, until the first supplying reducing agent to the NH ₃ synthesizing catalyst The period can be made longer than the supply interval of the second and subsequent reducing agents.

For example, when the NH ₃ production catalyst is an NOx storage reduction catalyst, a reducing agent is supplied to reduce the stored NOx. This reducing agent is supplied when the storage amount of NOx becomes equal to or greater than a threshold value, or is supplied at regular time intervals. Therefore, when the NOx occlusion amount becomes equal to or greater than the threshold after completion of the sulfur poisoning recovery, or when a specified time has elapsed, the reducing agent is supplied. When the air-fuel ratio is lowered by the supply of the reducing agent, H ₂ S is desorbed from the selective reduction type NOx catalyst and can be discharged without changing to SOx.

In contrast, the downstream recovery unit lengthens the period until the reducing agent is first supplied to the NH ₃ production catalyst. Thereby, the amount of oxygen flowing into the selective catalytic reduction NOx catalyst in a predetermined period after the recovery of the sulfur poisoning by the upstream side recovery unit is completed is determined from the amount of oxygen flowing into the selective catalytic reduction NOx catalyst after the predetermined period. Can also be more. Note that the reducing agent may not be supplied within a predetermined period.

In the present invention, the selective reduction NOx catalyst may include a base metal.

In such a selective reduction type NOx catalyst, H ₂ S can be adsorbed, and H ₂ S can be changed to SO x and discharged at the subsequent lean air-fuel ratio. The base metal may be, for example, Cu, Fe, or Ca. Thereby, since the maximum value of the amount of H ₂ S per unit time discharged downstream from the selective reduction type NOx catalyst can be reduced, generation of off-flavor can be suppressed.

According to the present invention, sulfur poisoning of both catalysts can be appropriately recovered when other catalysts are provided upstream of the selective reduction type NOx catalyst.

It is a figure which shows schematic structure of the internal combustion engine which concerns on an Example, its intake system, and an exhaust system. H 2 _S by mass, and is a time chart showing the mass of the progression of a combination of the SOx and H _{2 S.} When using Fe / MFI the SCR catalyst is a timing chart illustrating the changes of the _{H 2} S by weight. 6 is a time chart showing the transition of the air-fuel ratio and the mass of SOx flowing out from the NSR catalyst when performing the sulfur poisoning recovery process of the NSR catalyst. It is a time chart which showed transition of the air fuel ratio when sufficient time has passed since sulfur poisoning recovery processing of the NSR catalyst. 6 is a time chart showing the transition of the air-fuel ratio when the air-fuel ratio is 26 during a predetermined period immediately after completion of the sulfur poisoning recovery process of the NSR catalyst and when rich spike is not performed. 5 is a time chart showing the transition of the air-fuel ratio during a predetermined period immediately after completion of the sulfur poisoning recovery process of the NSR catalyst and when the intake air amount is 45 g / s when the rich spike is not performed. It is the flowchart which showed the flow of the air fuel ratio control which concerns on an Example.

Hereinafter, specific embodiments of the exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine according to the present embodiment and its intake system and exhaust system. The internal combustion engine 1 shown in FIG. 1 is a gasoline engine, but may be a diesel engine. The internal combustion engine 1 is mounted on a vehicle, for example.

The exhaust passage 2 is connected to the internal combustion engine 1. In the middle of the exhaust passage 2, in order from the upstream side, the three-way catalyst 3, the NOx storage reduction catalyst 4 (hereinafter referred to as NSR catalyst 4), and the selective reduction NOx catalyst 5 (hereinafter referred to as SCR catalyst 5). Is provided.

The three-way catalyst 3 purifies NOx, HC and CO with maximum efficiency when the catalyst atmosphere is at the stoichiometric air-fuel ratio. The three-way catalyst 3 has an oxygen storage capability. That is, by storing excess oxygen when the air-fuel ratio of the inflowing exhaust gas is a lean air-fuel ratio, and releasing the insufficient oxygen when the air-fuel ratio of the inflowing exhaust gas is rich, the exhaust gas is reduced. Purify.

By such an action of the oxygen storage ability, the three-way catalyst 3 can purify HC, CO, and NOx even if it is other than the stoichiometric air-fuel ratio. That is, by the action of the oxygen storage ability, it becomes possible to expand the range of the air-fuel ratio (also referred to as a purification window) in which the three-way catalyst 3 can purify HC, CO, and NOx at a predetermined ratio or more.

The NSR catalyst 4 stores NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and reduces the stored NOx when the oxygen concentration of the inflowing exhaust gas decreases and a reducing agent is present. It has a function. As the reducing agent supplied to the NSR catalyst 4, HC or CO that is unburned fuel discharged from the internal combustion engine 1 can be used.

Note that when the exhaust gas passes through the three-way catalyst 3 or the NSR catalyst 4, NOx in the exhaust gas may react with HC or H ₂ to generate ammonia (NH ₃ ). That is, in this embodiment, the three-way catalyst 3 or the NSR catalyst 4 corresponds to the NH ₃ production catalyst in the present invention. In the present embodiment, the three-way catalyst 3 or the NSR catalyst 4 is used as the NH ₃ production catalyst, but instead, NH ₃ can be produced and poisoned by sulfur components in the exhaust gas. Other exhaust purification catalysts may be used.

The SCR catalyst 5 has a function of adsorbing a reducing agent and selectively reducing NOx by the adsorbing reducing agent when NOx passes. The SCR catalyst 5 includes a base metal such as Cu or Fe in zeolite. For the SCR catalyst 5, for example, Cu / MFI or Fe / MFI can be used. As the reducing agent supplied to the SCR catalyst 5, NH ₃ produced by the three-way catalyst 3 or the NSR catalyst 4 can be used.

Further, in the exhaust passage 2 downstream of the three-way catalyst 3 and upstream of the NSR catalyst, a first temperature sensor 11 for detecting the temperature of the exhaust and an air-fuel ratio sensor 12 for detecting the air-fuel ratio of the exhaust are provided. It is attached. The temperature of the three-way catalyst 3 or the temperature of the NSR catalyst 4 can be detected by the first temperature sensor 11. Further, the air-fuel ratio sensor 12 can detect the air-fuel ratio of the exhaust gas from the internal combustion engine 1 or the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4.

Also, a second temperature sensor 13 for detecting the temperature of the exhaust is attached to the exhaust passage 2 downstream of the NSR catalyst 4 and upstream of the SCR catalyst 5. The temperature of the NSR catalyst 4 or the temperature of the SCR catalyst 5 can be detected by the second temperature sensor 13.

Also, a third temperature sensor 14 for detecting the temperature of the exhaust is attached to the exhaust passage 2 downstream of the SCR catalyst 5. The temperature of the SCR catalyst 5 can be detected by the third temperature sensor 14.

In addition, it is not necessary to attach all the above sensors, and they may be appropriately selected and attached.

Further, the internal combustion engine 1 is provided with an injection valve 6 for supplying fuel to the internal combustion engine 1.

On the other hand, an intake passage 7 is connected to the internal combustion engine 1. A throttle 8 for adjusting the intake air amount of the internal combustion engine 1 is provided in the middle of the intake passage 7. An air flow meter 15 that detects the intake air amount of the internal combustion engine 1 is attached to the intake passage 7 upstream of the throttle 8.

The internal combustion engine 1 configured as described above is provided with an ECU 10 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 10 controls the internal combustion engine 1 in accordance with the operating conditions of the internal combustion engine 1 and the driver's request.

In addition to the above sensors, the ECU 10 outputs an electric signal corresponding to the amount of depression of the accelerator pedal 16 by the driver to detect the engine load, and an accelerator position sensor 17 for detecting the engine speed. 18 are connected via electric wiring, and output signals of these various sensors are input to the ECU 10.

On the other hand, the injection valve 6 and the throttle 8 are connected to the ECU 10 via electric wiring, and the ECU 10 controls the opening / closing timing of the injection valve 6 and the opening degree of the throttle 8.

For example, the ECU 10 determines the required intake air amount from the accelerator opening detected by the accelerator opening sensor 17 and the engine speed detected by the crank position sensor 18. Then, the opening degree of the throttle 8 is controlled so as to realize the required intake air amount. The injection valve 6 is controlled so as to supply a fuel injection amount corresponding to the intake air amount that changes at this time. The air-fuel ratio set at this time is 25, for example, and is hereinafter referred to as a normal air-fuel ratio. This normal air-fuel ratio is an air-fuel ratio set according to the operating state of the internal combustion engine 1. Since the internal combustion engine 1 according to this embodiment is performing a lean burn operation except during rich spikes and during sulfur poisoning recovery, the normal air fuel ratio is a lean air fuel ratio.

Further, the ECU 10 performs a reduction process of NOx stored in the NSR catalyst 4. When NOx stored in the NSR catalyst 4 is reduced, the amount of fuel injected from the injection valve 6 or the opening of the throttle 8 is adjusted so that the air-fuel ratio of the exhaust gas flowing into the NSR catalyst 4 is reduced to a predetermined rich air-fuel ratio. A so-called rich spike is performed to lower it to a minimum.

This rich spike is carried out when the amount of NOx stored in the NSR catalyst 4 reaches a predetermined amount. The amount of NOx stored in the NSR catalyst 4 is calculated, for example, by integrating the difference between the amount of NOx flowing into the NSR catalyst 4 and the amount of NOx flowing out of the NSR catalyst 4. The amount of NOx flowing into the NSR catalyst 4 and the amount of NOx flowing out of the NSR catalyst 4 can be detected by attaching a sensor. Further, the rich spike may be performed every predetermined time or every predetermined traveling distance.

Further, in order to recover the NSR catalyst 4 from sulfur poisoning, the ECU 10 performs a sulfur poisoning recovery process of the NSR catalyst 4. In this sulfur poisoning recovery process, the temperature of the NSR catalyst 4 is raised to a temperature required for sulfur poisoning recovery (for example, 650 ° C. or higher), and then the air-fuel ratio is set to a predetermined rich air-fuel ratio (for example, 25). Is implemented. When the temperature of the NSR catalyst 4 is raised, the fuel injection amount from the injection valve 6 or the opening degree of the throttle 8 may be determined so that the lean air-fuel ratio is obtained.

The air-fuel ratio at the time of sulfur poisoning recovery and the air-fuel ratio at the time of NOx reduction (during rich spike) may be the same value or different values. Further, the air-fuel ratio at the time of rich spike may be set to a value equal to or lower than the theoretical air-fuel ratio and higher than 14.3. The air-fuel ratio at the time of sulfur poisoning recovery and the air-fuel ratio at the time of NOx reduction (during rich spike) are the air-fuel ratios within the purification window of the three-way catalyst 3. And a sulfur poisoning recovery process is performed for 10 minutes, for example. This time is set in advance as the time until the sulfur poisoning recovery process is completed. It is not necessary to release all sulfur components from the NSR catalyst 4.

Then, after the sulfur poisoning recovery process is completed, the normal air-fuel ratio is restored. Normal means when rich spike or sulfur poisoning recovery is not performed. Further, as described above, the normal air-fuel ratio is an air-fuel ratio that is set according to the operating state of the internal combustion engine 1, and is an air-fuel ratio when rich spike or sulfur poisoning recovery is not performed.

That is, the ECU 10 sets the air-fuel ratio to 25 when sulfur poisoning recovery or NOx reduction is not performed, and makes the air-fuel ratio rich when sulfur poisoning recovery or NOx reduction is performed.

By the way, when the SCR catalyst 5 is provided downstream of the NSR catalyst 4, NOx can be reduced by the SCR catalyst 5 even when sulfur poisoning of the NSR catalyst 4 occurs. For this reason, the decrease in the NOx purification rate of the exhaust purification device as a whole is smaller than when the SCR catalyst 5 is not provided. Here, even if sulfur poisoning occurs in the NSR catalyst 4, NH ₃ generated by the reaction of NOx and the reducing agent in the NSR catalyst 4 is used as the reducing agent in the SCR catalyst 5. That is, NOx is purified by the SCR catalyst 5 by NH ₃ . However, it has been found that when sulfur poisoning occurs in the NSR catalyst 4, not only the NOx storage ability is lowered, but also the NH ₃ production ability is lowered. Therefore, when sulfur poisoning of the NSR catalyst 4 occurs, the NOx purification rate in the SCR catalyst 5 may also decrease.

Here, when the sulfur poisoning of the NSR catalyst 4 is recovered, the ECU 10 controls the injection valve 6 or the throttle 8 so that the internal combustion engine 1 is operated at a rich air-fuel ratio of 14.3, for example. If the air-fuel ratio varies at this time and the air-fuel ratio becomes, for example, 14 or less, the sulfur component desorbed from the NSR catalyst 4 may change to H ₂ S. If this H ₂ S is directly released into the atmosphere, an unpleasant odor is generated, which is not preferable.

In contrast, in this embodiment, the SCR catalyst 5 is provided downstream of the NSR catalyst 4. The SCR catalyst 5 adsorbs H ₂ S. The H ₂ S adsorbed by the SCR catalyst 5 is desorbed from the SCR catalyst 5 when the internal combustion engine 1 is operated at a lean air-fuel ratio, is changed to SOx by oxygen in the exhaust gas, and is released into the atmosphere.

Here, FIG. 2 is a time chart showing the transition of the H ₂ S mass and the total mass of SOx and H ₂ S. The lean air-fuel ratio is before the time indicated by A, and the air-fuel ratio is, for example, a rich air-fuel ratio of 12, after the time indicated by A. The air-fuel ratio after the time point indicated by A is the richer air-fuel ratio than when the sulfur poisoning recovery process is performed, and the air-fuel ratio for actively generating H ₂ S for the experiment is The fuel ratio. The solid line indicates the H ₂ S mass per unit time flowing out from the SCR catalyst 5. The alternate long and short dash line indicates the H ₂ S mass per unit time flowing into the SCR catalyst 5. Note that the dashed line may be a H _{2 S} mass per unit of efflux time from the NSR catalyst 4, as H _{2 S} mass per unit time passing through the exhaust passage 2 in the case without the SCR catalyst 5 Good. The two-dot chain line indicates the total mass of SOx and H ₂ S flowing out from the NSR catalyst 4 per unit time. Here, Cu / MFI is used for the SCR catalyst 5.

In FIG. 2, the difference between the two-dot chain line and the one-dot chain line can be the mass of SO ₂ and SO ₃ . In FIG. 2, the difference between the alternate long and short dash line and the solid line can be the mass of H ₂ S adsorbed by the SCR catalyst 5. That is, when the exhaust gas passes through the SCR catalyst 5, the mass of H ₂ S decreases. It is considered that the decrease in H ₂ S is adsorbed by the SCR catalyst 5. Thus, it has been found that when SCR catalyst 5 including Cu such as Cu / MFI is used, not only NH ₃ but also H ₂ S is adsorbed.

Next, FIG. 3 is a time chart showing transition of H ₂ S mass when Fe / MFI is used for the SCR catalyst 5. The meanings of the solid line, the one-dot chain line, and the two-dot chain line are the same as those in FIG.

In FIG. 3, comparing the solid line with the one-dot chain line, it can be seen that the maximum value of the solid line is smaller than the maximum value of the one-dot chain line. That is, when the exhaust gas passes through the SCR catalyst 5, the maximum value of H ₂ S mass decreases. This means that H ₂ S is adsorbed on the SCR catalyst 5. However, after the solid line reaches the maximum value, it can be seen that the solid line is larger than the one-dot chain line. That is, after the H ₂ S mass reaches the maximum value, the H ₂ S mass downstream of the SCR catalyst 5 is larger than the upstream H ₂ S mass. Thus, when the solid line is larger than the one-dot chain line, it is considered that H ₂ S is desorbed from the SCR catalyst 5.

That is, when the SCR catalyst 5 including Fe such as Fe / MFI is used, even if H ₂ S is adsorbed to the SCR catalyst 5, this H ₂ S is considered to be desorbed after that. . For this reason, the effect of reducing the mass of H ₂ S flowing out from the SCR catalyst 5 is small. However, since the maximum value of the H ₂ S mass can be reduced by the SCR catalyst 5 once adsorbing H ₂ S, there is an effect of suppressing the off-flavor.

The H ₂ S adsorbed on the SCR catalyst 5 becomes SOx due to oxygen in the exhaust gas and flows out of the SCR catalyst 5 when the internal combustion engine 1 is operated at a lean air-fuel ratio. Thereby, generation | occurrence | production of a strange odor can be suppressed.

By the way, when there are many opportunities to perform the sulfur poisoning recovery process of the NSR catalyst 4, the possibility that H ₂ S is generated increases. In addition, whenever H ₂ S is adsorbed to the SCR catalyst 5 every time the sulfur poisoning recovery process of the NSR catalyst 4 is performed, NH ₃ is less likely to be adsorbed to the SCR catalyst 5. That is, in the SCR catalyst 5, the pores to adsorb NH _3, and the previously would adsorb the _{H 2} S than NH _3, can not be adsorbed to NH _3. If the amount of H ₂ S adsorbed on the SCR catalyst 5 increases, NH ₃ for NOx reduction may be insufficient, and the NOx purification rate may decrease.

Therefore, in this embodiment, after the sulfur poisoning recovery process of the NSR catalyst 4 is performed, H ₂ S is desorbed from the SCR catalyst 5. Here, by supplying a large amount of oxygen to the SCR catalyst 5, H ₂ S adsorbed by the SCR catalyst 5 can be discharged as SOx. For example, immediately after completion of the sulfur poisoning recovery process of the NSR catalyst 4, the air-fuel ratio is set to 26, and after the time required for removing H ₂ S from the SCR catalyst 5 has elapsed, the air-fuel ratio is set to the normal 25. . That is, the air-fuel ratio immediately after completion of the sulfur poisoning recovery process of the NSR catalyst 4 is made higher than the normal air-fuel ratio. Further, for example, the intake air amount of the internal combustion engine 1 between the completion of the sulfur poisoning recovery process of the NSR catalyst 4 and the time when the first rich spike is performed is increased from that after the first rich spike is performed. Thus, the SV of the exhaust gas passing through the SCR catalyst 5 may be increased. As a result, the amount of oxygen flowing into the SCR catalyst 5 in a predetermined period increases, so that H ₂ S can be quickly desorbed as SOx.

Further, the interval from the completion of the sulfur poisoning recovery process of the NSR catalyst 4 to the first rich spike may be made longer than the interval between the second and subsequent rich spikes. That is, by increasing the time until the first rich spike is performed, more oxygen can be supplied to the SCR catalyst 5, so that H ₂ S can be quickly desorbed as SOx. Further, for example, the rich spike may be prohibited until the time required for removing H ₂ S elapses after the sulfur poisoning recovery process of the NSR catalyst 4 is completed.

Here, FIG. 4 is a time chart showing changes in the air-fuel ratio and the mass of SOx flowing out from the NSR catalyst 4 when the sulfur poisoning recovery process of the NSR catalyst 4 is being performed. The sulfur poisoning recovery process is performed for approximately 10 minutes. The air-fuel ratio at this time is 14.3, for example. When the sulfur poisoning recovery process is completed, the mass of SOx flowing out from the NSR catalyst 4 is relatively small. Thereafter, H ₂ S is removed from the SCR catalyst 5.

FIG. 5 is a time chart showing the transition of the air-fuel ratio when a sufficient time has elapsed from the sulfur poisoning recovery process of the NSR catalyst 4. The air / fuel ratio may be a value detected by a sensor or a target value set by the ECU 10. In FIG. 5 and FIGS. 6 and 7 described later, when the time is 0, the sulfur poisoning recovery process of the NSR catalyst 4 is completed. FIG. 5 shows the transition of the air-fuel ratio when the SCR catalyst 5 is hardly adsorbed with H ₂ S because a sufficient time has elapsed since the sulfur poisoning recovery process of the NSR catalyst 4. That is, it is the transition of the air-fuel ratio when the control for desorbing H ₂ S is not performed. In FIG. 5, the air-fuel ratio when the rich spike is not performed is, for example, 25, and the intake air amount at this time is, for example, 40 g / s. The air-fuel ratio at this time is the normal air-fuel ratio.

FIG. 6 is a time chart showing the transition of the air-fuel ratio when the air-fuel ratio is 26 during a predetermined period immediately after the completion of the sulfur poisoning recovery process of the NSR catalyst 4 and when the rich spike is not performed. That is, the air-fuel ratio is made higher than when sufficient time has elapsed since the sulfur poisoning recovery process of the NSR catalyst 4. The period during which the air-fuel ratio is increased can be obtained in advance by experiments or the like as the period required to remove H ₂ S from the SCR catalyst 5. The intake air amount is the same as that in FIG. In this way, by increasing the air-fuel ratio during a predetermined period, more oxygen is supplied to the SCR catalyst 5, so that H ₂ S can be desorbed as SOx.

FIG. 7 shows a predetermined period immediately after the completion of the sulfur poisoning recovery process of the NSR catalyst 4, and shows the transition of the air-fuel ratio when the intake air amount is 45 g / s when the rich spike is not performed. It is a chart. That is, the amount of intake air is increased compared to when sufficient time has elapsed since the sulfur poisoning recovery process of the NSR catalyst 4. The period during which the amount of intake air is increased can be obtained in advance through experiments or the like as the period required to remove H ₂ S from the SCR catalyst 5. Further, the period during which the intake air amount is increased may be a period from when the sulfur poisoning recovery process of the NSR catalyst 4 is completed to when the first rich spike is performed. The air-fuel ratio is 25, the same as in FIG. In this way, by increasing the amount of intake air during a predetermined period, more oxygen is supplied to the SCR catalyst 5, so that H ₂ S can be desorbed as SOx.

In this way, it is possible to supply more oxygen to the SCR catalyst 5 by increasing the air-fuel ratio or increasing the intake air amount. Thereby, H ₂ S can be quickly desorbed as SOx. In addition, since the SCR catalyst 5 can adsorb NH ₃ required for NOx reduction, the NOx purification rate can be increased. The air-fuel ratio can be changed by adjusting the amount of fuel injected from the injection valve 6 or the opening of the throttle 8. The intake air amount can be changed by adjusting the opening of the throttle 8. If the air-fuel ratio is not changed when increasing the intake air amount, the fuel injection amount is increased according to the intake air amount.

Next, FIG. 8 is a flowchart showing a flow of air-fuel ratio control according to the present embodiment. This routine is executed by the ECU 10 every predetermined time.

In step S101, it is determined whether it is time to perform the sulfur poisoning recovery process. This sulfur poisoning recovery process is a process for recovering the sulfur poisoning of the NSR catalyst 4. In this step, it may be determined whether or not the NOx purification rate in the NSR catalyst 4 may be less than the allowable range due to the SOx stored in the NSR catalyst 4. Here, it may be difficult to detect the mass of SOx stored in the NSR catalyst 4. Therefore, for example, it may be determined that it is time to perform the sulfur poisoning recovery process when the operation time of the internal combustion engine 1 has exceeded a specified time since the previous sulfur poisoning recovery process. Alternatively, it may be determined that it is time to perform the sulfur poisoning recovery process every time the travel distance of the vehicle on which the internal combustion engine 1 is mounted reaches a specified distance. Further, for example, the amount of SOx stored in the NSR catalyst 4 may be estimated, and it may be determined whether or not the amount of SOx is greater than or equal to a threshold value. The amount of SOx may be estimated by a known technique.

If an affirmative determination is made in step S101, the process proceeds to step S102. On the other hand, if a negative determination is made in step S101, it is not necessary to perform the sulfur poisoning recovery process of the NSR catalyst 4, and thus this routine is terminated.

In step S102, a sulfur poisoning recovery process is performed on the NSR catalyst 4. First, the temperature of the NSR catalyst 4 is raised to a temperature required for sulfur poisoning recovery (for example, 650 ° C.). Note that when the temperature of the NSR catalyst 4 is increased, a lean air-fuel ratio may be set. Thereafter, a predetermined rich air-fuel ratio (for example, 14.3) suitable for releasing SOx is set. In this embodiment, the ECU 10 that processes step S102 corresponds to the upstream recovery unit in the present invention.

In step S103, it is determined whether the sulfur poisoning recovery process of the NSR catalyst 4 is completed. For example, when the elapsed time from the start of the sulfur poisoning recovery process of the NSR catalyst 4 exceeds a specified time, it is determined that the sulfur poisoning recovery process of the NSR catalyst 4 has been completed. If a positive determination is made in step S103, the process proceeds to step S104. On the other hand, if a negative determination is made in step S103, the process returns to step S102, and the sulfur poisoning recovery process of the NSR catalyst 4 is continued.

In step S104, an H ₂ S removal process is performed. The H ₂ S removal process is a process for removing H ₂ S from the SCR catalyst 5. This can be said to be a process for recovering sulfur poisoning of the SCR catalyst 5. In this step, a process for quickly releasing H ₂ S adsorbed by the SCR catalyst 5 is performed. That is, the air-fuel ratio is increased or the intake air amount is increased. The time from the completion of the sulfur poisoning recovery process of the NSR catalyst 4 to the first rich spike may be extended. That is, more oxygen may be supplied to the SCR catalyst 5 before performing the first rich spike. Here, the rich spike while the SCR catalyst 4 are adsorbed H _{2 S} is carried out, since the H 2 _S there is a fear that is released atmosphere without change to SOx, suppresses this. The H ₂ S removal process may be performed continuously with the sulfur poisoning recovery process of step S102 or may be performed at intervals. In this embodiment, the ECU 10 that processes step S104 corresponds to the downstream recovery unit in the present invention.

In step S105, it is determined whether the H ₂ S removal process is completed. For example, when the elapsed time since the start of the H ₂ S removal process is equal to or longer than a specified time, it is determined that the H ₂ S removal process is completed. This predetermined time is obtained by experiments or the like. If an affirmative determination is made in step S105, the process proceeds to step S106. On the other hand, if a negative determination is made in step S105, the process returns to step S104, and the H ₂ S removal process is continued.

In step S106, normal control is performed. Here, the normal control means that the rich spike is performed when the internal combustion engine 1 is operated at a normal air-fuel ratio and the amount of NOx stored in the NSR catalyst 4 exceeds a threshold value.

As described above, according to this embodiment, H ₂ S can be removed from the SCR catalyst 5 immediately after the sulfur poisoning recovery process of the NSR catalyst 4 is performed. Thereby, the fall of a NOx purification rate can be suppressed.

Moreover, since the ability to generate NH ₃ in the NSR catalyst 4 is recovered by the sulfur poisoning recovery process, it is possible to suppress a shortage of the reducing agent in the SCR catalyst 5, and thus it is possible to suppress a decrease in the NOx purification rate.

Further, H ₂ S can be adsorbed on the SCR catalyst 5 and then discharged as SOx. Therefore, it is possible to reduce the concentration of H _{2 S} which flows out to the downstream side of the SCR catalyst 5, the generation of an odor can be suppressed.

By the way, in the case of the SCR catalyst 5 including Cu such as Cu / MFI, Cu and H ₂ S react to become copper sulfate. Since this copper sulfate is water-soluble, Cu may be lost from the SCR catalyst 5 due to moisture in the exhaust. On the other hand, by removing H ₂ S promptly, loss of Cu from the SCR catalyst 5 can be suppressed, so that a reduction in the NOx purification rate can be suppressed.

Thus, according to the present embodiment, sulfur poisoning of the NSR catalyst 4 and the SCR catalyst 5 can be properly recovered.

In the present embodiment, the temperature decrease of the NSR catalyst 4 or the SCR catalyst 5 is suppressed after the sulfur poisoning recovery process of the NSR catalyst 4 is completed. Here, when the sulfur poisoning recovery process of the NSR catalyst 4 is performed, the temperature of the NSR catalyst 4 is raised. That is, the temperature of the NSR catalyst 4 becomes higher than before the sulfur poisoning recovery process is performed. And if the temperature of the NSR catalyst 4 becomes high, the temperature of the SCR catalyst 5 downstream will also become high. Thereby, the desorption of H ₂ S from the SCR catalyst 5 is promoted. Since other devices are the same as those of the first embodiment, the description thereof is omitted.

In this embodiment, the temperature of the NSR catalyst 4 or the SCR catalyst 5 when the sulfur poisoning recovery process of the NSR catalyst 4 is completed may be maintained as it is. Further, the rate at which the temperature of the NSR catalyst 4 or the SCR catalyst 5 is lowered may be decreased. Further, the temperature of the NSR catalyst 4 or the SCR catalyst 5 may be maintained at a higher temperature than before the sulfur poisoning recovery process of the NSR catalyst 4 is performed. Further, the temperature of the NSR catalyst 4 or the SCR catalyst 5 may be made higher than when the sulfur poisoning recovery process of the NSR catalyst 4 is completed. Moreover, after the sulfur poisoning recovery process of the NSR catalyst 4 is completed and the temperature of the NSR catalyst 4 or the SCR catalyst 5 is lowered, it may be raised again.

For example, the temperature of the NSR catalyst 4 or the SCR catalyst 5 may be set to a temperature sufficient for desorbing H ₂ S from the SCR catalyst 5 (for example, 650 ° C.). For example, when the sulfur poisoning recovery process of the NSR catalyst 4 is not performed, the NSR catalyst 4 may be maintained at 450 ° C. which is higher than that in the operation state where the temperature of the NSR catalyst 4 becomes 420 ° C.

Note that a decrease in the temperature of the NSR catalyst 4 or the SCR catalyst 5 can be suppressed by increasing the fuel injection amount or changing the valve timing or the valve lift amount. Further, the NSR catalyst 4 or the SCR catalyst 5 may be heated using an electric heater. Further, for example, when an EGR device is provided, a decrease in the temperature of the NSR catalyst 4 or the SCR catalyst 5 can be suppressed by stopping the supply of EGR gas. Further, the temperature drop of the NSR catalyst 4 or the SCR catalyst 5 can be suppressed by making the air-fuel ratio lower than the normal air-fuel ratio or delaying the ignition timing.

In this way, a decrease in the temperature of the SCR catalyst 5 can be suppressed. Here, the higher the temperature of the SCR catalyst 5, the more easily H ₂ S is desorbed from the SCR catalyst 5. Then, if the stoichiometric air-fuel ratio or a lean air-fuel ratio, since the H 2 _S is changed to SOx, it is possible to prevent the H _{2 S} is discharged into the atmosphere. If the control for supplying more oxygen to the SCR catalyst 5 as described in the first embodiment is used in combination, H ₂ S can be discharged as SOx more quickly.

In addition, although the present Example demonstrates the example which applied the exhaust gas purification apparatus to the gasoline engine, it can also be applied to a diesel engine. When applied to diesel engines, at least one of main injection amount change, main injection timing change, after injection addition, after injection timing change, post injection addition, post injection timing change is implemented. By doing so, the fall of the temperature of the NSR catalyst 4 or the SCR catalyst 5 can be suppressed. In the case of a diesel engine, fuel may be added to the exhaust gas, or the NSR catalyst 4 or the SCR catalyst 5 may be heated by a burner or an electric heater.

In the present embodiment, in step S104 of the flow shown in FIG. 8 of the first embodiment, a decrease in the temperature of the NSR catalyst 4 or the SCR catalyst 5 is suppressed as the H ₂ S removal process. At this time, the process of step S104 described in the first embodiment may be performed together. In this embodiment, the ECU 10 that processes step S104 corresponds to the downstream recovery unit in the present invention.

As described above, according to this embodiment, it is possible to quickly desorb H ₂ S from the SCR catalyst 5 by suppressing the temperature that has risen during the recovery of sulfur poisoning of the NSR catalyst 4 from decreasing thereafter. it can. Thereby, the fall of a NOx purification rate can be suppressed.

Moreover, since the ability to generate NH ₃ in the NSR catalyst 4 is recovered by the sulfur poisoning recovery process, it is possible to suppress a shortage of the reducing agent in the SCR catalyst 5, and thus it is possible to suppress a decrease in the NOx purification rate.

Further, H ₂ S can be adsorbed on the SCR catalyst 5 and then discharged as SOx. Therefore, it is possible to reduce the concentration of H _{2 S} which flows out to the downstream side of the SCR catalyst 5, the generation of an odor can be suppressed.

Furthermore, since it is possible to suppress the loss of Cu from the SCR catalyst 5, it is possible to suppress a reduction in the NOx purification rate.

Thus, according to the present embodiment, sulfur poisoning of the NSR catalyst 4 and the SCR catalyst 5 can be properly recovered.

1 Internal combustion engine 2 Exhaust passage 3 Three-way catalyst 4 NOx storage reduction catalyst (NSR catalyst)
5 Selective reduction type NOx catalyst (SCR catalyst)
6 Injection valve 7 Intake passage 8 Throttle 10 ECU
11 First temperature sensor 12 Air-fuel ratio sensor 13 Second temperature sensor 14 Third temperature sensor 15 Air flow meter 16 Accelerator pedal 17 Accelerator opening sensor 18 Crank position sensor

Claims (7)

And NH ₃ synthesizing catalyst to produce a NH ₃ provided in an exhaust passage of an internal combustion engine,
A selective reduction type NOx catalyst that is provided in an exhaust passage downstream of the NH ₃ production catalyst and selectively reduces NOx;
An upstream recovery portion for recovering sulfur poisoning of the NH ₃ production catalyst;
A downstream recovery unit that recovers sulfur poisoning of the selective reduction type NOx catalyst after the sulfur recovery of the NH ₃ production catalyst is recovered by the upstream recovery unit;
An exhaust gas purification apparatus for an internal combustion engine. The downstream recovery unit recovers the sulfur poisoning of the NH ₃ production catalyst when the temperature of the selective reduction type NOx catalyst that has risen when the upstream recovery unit restores the sulfur poisoning of the NH ₃ production catalyst. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the exhaust gas purification device is prevented from descending after being caused to fall. The downstream recovery unit is configured to reduce an oxygen amount flowing into the selective reduction type NOx catalyst in a predetermined period after the sulfur poisoning of the NH ₃ generation catalyst is recovered by the upstream recovery unit after the predetermined period. The exhaust gas purification apparatus for an internal combustion engine according to claim 1 or 2, wherein sulfur poisoning is recovered by increasing the amount of oxygen flowing into the selective reduction type NOx catalyst. 4. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the downstream side recovery unit raises the air-fuel ratio of the exhaust gas in the predetermined period higher than in the predetermined period. The exhaust purification device for an internal combustion engine according to claim 3 or 4, wherein the downstream recovery unit increases the intake air amount of the internal combustion engine in the predetermined period more than in the predetermined period and thereafter. The downstream recovery unit has a period from the recovery of the sulfur poisoning of the NH ₃ generation catalyst by the upstream recovery unit to the first supply of the reducing agent to the NH ₃ generation catalyst. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 3 to 5, wherein the exhaust gas purification apparatus is longer than a supply interval of the reducing agent. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 6, wherein the selective reduction type NOx catalyst includes a base metal.

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